In a semiconductor laser which is applied as an optical disc system, noise induced by optical feedback from recording material to the laser facet causes a problem. Accordingly, in order to obtain a stable S/N (signal-to-noise ratio), it is proposed to control the front facet reflectivity of a semiconductor laser to have a value within 21.+-.3%. FIG. 4 shows typical characteristics of S/N versus optical feedback ratio in a case where the front facet reflectivity is 20%.
FIG. 5 shows characteristics of the facet reflectivity versus film thickness in a case where a single layer film of Al.sub.2 O.sub.3 is used for facet protection film as is generally used. As is apparent from FIG. 5, it is required to control the film thickness to achieve the desired reflectivity. In order to make the facet reflectivity within 21.+-.3%, the optical thickness of the Al.sub.2 O.sub.3 layer must lie in one of two ranges marked a in FIG. 5 that lie on opposite sides of the quarter wavelength optical thickness which is at the center of of FIG. 5.
In the prior art semiconductor laser having a facet protection film of such a construction, the facet reflectivity of 21.+-.3% is realized only by controlling the thickness of the single layer film of Al.sub.2 O.sub.3. However, since any variation in the facet reflectivity which occurs is largely dependent on film thickness, as is apparent from FIG. 5, a facet reflectivity of 21.+-.3% cannot be obtained due to the variations produced in film thickness during deposition of the film.